FTIR INVESTIGATION OF OT-CENTER-DOT-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O HYDROGEN-BONDS WITH LARGE PROTON POLARIZABILITY IN SULFONIC ACID-N-OXIDE SYSTEMS IN THE MIDDLE AND FAR-IR

Sixteen 1 : 1 methanesulfonic acid-N-oxide systems were studied as a f unction of the basicity of the N-oxides. The observed IR continua demo nstrate that a single minimum proton potential is shifted, with increa sing basicity, from the acid to the N-oxide. The largest proton polari zability is attained with the system that shows the (on average) most symmetrical proton potential, as indicated by the maximum bathochromic shift of the IR continuum. This shift toward lower wavenumbers is lar gest with the methanesulfonic acid-3,5-dichloropyridine-N-oxide comple x, for which the integrated absorbance of the IR continuum is also lar gest and the continuous absorption is nearly temperature independent. The significant intensity distribution at the symmetry point indicates very strong and short O ... H ... O hydrogen bonds. The proton transf er was studied via the SO-stretching vibration bands. The steady shift of the acid to the acid anion SO bands reflects a continuous proton t ransfer process within this family of systems. Study of the far-IR reg ion shows that, especially in the most symmetrical cases, the IR conti nua extend down to 100 cm(-1) or less. Semi-empirical calculations sho w that the observed hydrogen-bond vibrations differ largely from hydro gen-bond cm stretching vibrations (v(sigma)) and have complicated vibr ational character (v(HB)). In contrast to the methanesulfonic acid-sul foxide (phosphine oxide, arsine oxide) family of systems no broadening of this transition occurs. The position of the bands indicates, howev er, a significant trend of the force constant. It is shown that the sy stem with the strongest hydrogen bonds is that with the largest bathoc hromic shift of the IR continuum, i.e. the system with the largest pro ton polarizability.